FIG. 1 of the accompanying drawings shows a standard starter circuit diagram.
The electrical starter motor M is connected between ground and a terminal +Bat at the battery supply voltage.
Energisation of the motor M is controlled by a contactor C which is a relay comprising a power contact 3 controlled by a latching coil 1 and a pick-up coil 2.
The power contact 3 is disposed between the motor M and the supply terminal at the voltage +Bat, for example.
A common end of the pick-up and latching coils 1 and 2 is connected to the +Bat supply terminal, for example via a starter switch 4 which is generally an ignition switch. The opposite end of the latching coil 1 is connected to ground and the pick-up coil 2 is connected to a point between the contact 3 and the motor M.
When the ignition switch 4 closes, the two coils 1 and 2 are energised simultaneously and their magnetic forces of attraction on the mobile core of the contactor add together. The attraction forces are sufficient to overcome the return springs and the friction on the contactor and on the starter. At the end of its travel the closing of the power contact 3 applies substantially the same potential to both ends of the pick-up coil 2, which prevents any current flowing through it. Only the latching coil 1 remains energised. However, because of the very small air gap that remains at this time, the forces generated by the holding coil 1 remain higher than the return forces of the various springs, which means that the contactor C can remain closed. This economises the current consumed by the pick-up coil 2 and prevents it overheating.
Electronic control of the contactor enables the use of only one coil. This is shown by the circuit represented in FIG. 2, in which the power contact 3 of the contactor C is moved by an energisation coil B connected between ground and the supply terminal +Bat at the battery voltage. Energisation of the coil B is controlled by a control unit U which operates a switch S. The control unit U is generally a microprocessor one input e of which is connected to the +Bat terminal via the starter switch 4, for example, and an output s of which controls the switch S, which is a MOSFET, for example.
When the switch 4 closes the microprocessor U carries out a number of operations to assure that the starter is ready to be actuated, whereupon the transistor S is commanded by a pulse width modulation (PWM) signal to generate at the coil B a predetermined voltage law to assure forward movement of the mobile core at low speed.
FIG. 3a shows a sequence of closing the starter switch 4 and FIG. 3b shows one example of the evolution in time of the average energisation current Ic flowing in the coil B during the closing sequence. FIG. 3c shows the closing sequence of the power contact 3 that corresponds to this energisation.
Throughout a first period T following the closing of the switch 4 the current Ic is maintained at a sufficiently high pick-up level to guarantee that the power contact 3 is closed. The period T is made sufficiently long for the contacts to close in all operating configurations: battery type, battery charge state, starter type, starting temperature, etc.
At the end of this first period the PWM control function of the microprocessor U reduces the current in the coil B to a minimal value im which holds the magnetic circuit closed.
The reader will already have understood that the uncertain nature of the time actually required to close the power contact 3 imposes an overgenerous time T for the change to latching mode.
However, in most cases the power contact 3 has closed well before the end of the period T (between the times that correspond to the points A and B shown in FIG. 3b).
This causes unnecessary overheating of the power transistor 3 throughout the portion of the period T in which the power contact 3 is closed, i.e. throughout the period D shown in FIG. 3c.